How can you tell whether a carton with a screw cap has already been opened?

On whether the foil is the safety feature people think — the honest answer is that the seal is not the primary protection, and it never was.

A determined person can get into almost any package. Cartons in particular can be entered from the base or a seam and resealed. So the foil is not a barrier against a serious tampering attempt; it is a tell that makes casual interference obvious and that guarantees the contents were sealed at the factory.

The actual protections are the supply chain, the packaging integrity as a whole, and the fact that food tampering is rare and heavily investigated.

So the practical advice is to look at the whole package rather than fixating on the foil: unusual weight, a soft or bulging carton, leakage, a bad smell on opening, dates that do not make sense. Any of those is a stronger signal than the seal.

The most reliable indicator is not the seal at all, it is the pressure.

Give the carton a gentle squeeze. An intact carton resists — you can compress it slightly and then it pushes back, because the contents and the small headspace are enclosed. A carton whose seal is broken lets air out as you squeeze, so it keeps giving and does not spring back.

That test works through the packaging without opening anything, takes two seconds, and is much harder to defeat than any visual check.

Secondary checks: the cap's tamper ring, if it has one, should be intact; the foil should be flat and undisturbed rather than dished or wrinkled; and the carton should show no puncture marks, particularly near the corners and the base seam.

One thing worth noting about these cartons specifically: they are usually aseptically packed, which is why they sit on a shelf rather than in a chiller.

The contents were sterilised and filled in a sterile environment, so an unopened carton is stable at room temperature for a long time. Once opened, that protection is gone and it becomes an ordinary perishable — refrigerate and use within a few days.

That matters for your question because the difference between opened and unopened is not cosmetic. An opened carton that has been sitting at room temperature is genuinely a different proposition, and it is the reason the question is worth asking rather than being merely fastidious.

For anything you buy that you are uncertain about: the shop will replace it without argument.

Retailers deal with this constantly and would far rather exchange a carton than have a customer take a chance. If the squeeze test is ambiguous and you are not comfortable, it is a two-minute conversation and no reasonable shop makes it difficult.